1. Field of the Invention
This invention is related to energizing a discharge device, such as lamps producing a light flash of intense brilliance. More specifically, the present invention provides a circuit for producing sufficient current to energize a flashlamp and to sustain the energization of the flashlamp for a desired period of time. The present invention also provides a method for energizing a flashlamp by providing a brief high-voltage pulse to energize a flashlamp, and a longer lower-voltage pulse to sustain it. This increases the life of the flashlamp by minimizing the time for which an unnecessarily high current passes between its electrodes.
2. Description of the Prior Art
In flashlamp circuits, lamp life of a flashlamp is an important factor. The number of flashes that a flashlamp can produce at acceptable light levels is one important parameter of its performance. A typical flashlamp circuit, such as the one shown in FIG. 1 and as further discussed below, includes a charging circuit to convert an available voltage to a higher voltage as required to operate the flashlamp, a capacitor to hold an electrical charge as it is developed and to supply it in one large, fast surge once the flashlamp is triggered, and a trigger transformer to initiate ionization of gas contained in the flashlamp. To initiate a flash, a high voltage is applied to the outside surface of the flashlamp. This allows current to start to flow within the lamp, provided that there is sufficient voltage between the electrodes of the flashlamp. Once current flow has started, the flashlamp has a nonlinear but positive resistance; the higher the voltage, the higher the current. In such a conventional circuit, the current begins at a high value (for a small camera flashlamp, this can be 200-300 amperes). As current flows, the capacitor discharges, its voltage falls, and the current consequently falls into the 50-150 amp range. Finally, there is insufficient current to sustain the discharge and the process ceases. The high-current initial transient is necessary because the flashlamp requires a high voltage between its terminals to start the arc. The shape of the curve, then, is determined by the V/I curve of the flashlamp as applied against the size of the holding capacitor, modified by whatever other components and parasitics lie along the path (e.g. gating transistor or SCR, shaping inductors, etc.)
In such conventional flashlamp circuits, flashlamps may be damaged in several ways. The envelope of the flashlamp can develop a set of fine cracks due to repeated acoustic shocks from discharge. Also, vaporization of the electrodes of the flashlamp may degrade their shapes, or the material of the electrodes may be vaporized and redeposited on the inside of the envelope, causing irregular darkening of the envelope.
In response to these problems, the prior art has provided a number of solutions for extending the lamp life of a flashlamp. The use of a more durable material, such as quartz, for the envelope can make it more resistant to cracking due to the repeated acoustic shocks from discharge. Complex electrode formulations and fabrication also increase life. Both of these solutions however come at added cost. Another solution for extending the lamp life of a flashlamp include reducing the amount of energy passed through the flashlamp at each flash. This causes a concomitant problem of reduced flash light output. Another solution for extending the lamp life is to smear and shape the initial high current part of the capacitor discharge with an inductor. This is usually of limited utility, since an inductor must be both large and efficient at high currents; typically, 100 microhenries or greater in order to function well at 300 amps. Such an inductive device will be large and expensive.
Therefore, what is needed and what has been invented is a flashlamp circuitry and method of prolonging the lamp life of a flashlamp wherein more flashes may be achieved, especially with a low-cost bulb, without compromising the light output of the flashlamp.